A cell wall fraction of the yeast phase cells of Blastomyces dermatitidis has proven to be sensitive and specific in experimental blastomycosis. The guinea pig model has been used for the evaluation of this antigen in in vivo skin testing, in in vitro assays for migration inhibition (MIF), and lymphocyte transformation of lymph node homogenates and peripheral blood lymphocytes. The antigen has been separated into two biologically active components by isoelectric focusing and evaluated by skin testing and lymphocyte transformation. These components proved to be less reactive than the unfractionated antigen. Antigens prepared from three different isolates of Blastomyces dermatitidis are currently being compared in an experimental model.